This invention relates to circuitry for determining the position of a motor driven actuator by identifying and counting motor commutation pulses.
It is frequently desired, either for control or diagnostic purposes, to detect the position of a motor driven actuator, such as a movable door in an automotive air conditioning duct. In applications where the actuator is driven by a brush-type DC motor, the actuator position may be reliably and inexpensively determined by detecting and counting pulses in the motor current caused by the periodic commutation of motor current by the motor brushes. In general, the pulses are extracted by filtering, amplified, and then counted to form a count corresponding to the actuator/motor position.
Several different pulse detection circuits have been proposed. For example, the U.S. Pat. No. 5,514,977 references a circuit having a resistive shunt connected in series with the motor. In such case, the voltage at a node of the resistive shunt is capacitively coupled to a filter circuit.
A potential for error exists in the above-described circuits due to a sudden voltage rise in the motor drive circuit wires at motor turn on. Specifically, the sudden voltage rise results in a detected pulse (referred to herein as a start-up pulse) that may be mistaken for a commutation pulse, resulting in a motor position indication error. Accordingly, what is needed is a simple detection circuit that rejects start-up pulses, but that reliably detects all commutation pulses.
The present invention is directed to improved motor position detection circuitry based on commutation pulse counting, the circuitry including a pulse recognition circuit, a pulse counting circuit, and a start pulse rejection circuit for rendering the position detection circuitry insensitive to start-up pulses associated with motor turn-on. The start pulse rejection circuit detects an off-to-on transition of a motor controller, and renders the position detection circuitry insensitive to start-up pulses generated in a predefined time window coinciding with the detected off-to-on transition.
According to a first embodiment, the start pulse rejection circuit maintains a nominal bias voltage at an input of the pulse recognition circuit for the duration of the predefined time window. According to a second embodiment, the start pulse rejection circuit renders the pulse counting circuit insensitive to pulses produced by the pulse recognition circuit for the duration of the predefined time window.